By government mandate, most dry products must be sold by weight rather than volume. Most granular and powdered products vary in density to such an extent that dispensing equipment, which is usually of the rotating auger type, must be constantly monitored in order to assure that no package has less weight than that specified on the label.
Elaborate means, such as downstream check-weighers, have been devised to remove underweight packages from the conveyor system. In some filling systems, the check-weighers provide feedback to the dispensing equipment, such as an adjustment to the rotational travel of the auger. Such a method lacks accuracy, since the check-weigher is downstream and density of the product may be varying rapidly. In any case, adjustment is made only after an error has been detected.
Another method places the container on a weight scale as it is being filled. The scale automatically stops the auger when the proper weight is attained. Although some error arises as the product continues to fall from the auger as the proper weight is reached, accuracy over the check-weigher feedback system is substantially improved. However, not only is the filling process lengthened considerably, frequent calibration of the weight scale and adjustment of the filling equipment is necessary. Such weight-measurement systems are also impractical for certain types of containers such as envelopes, which must be held in position during filling.
A later development has been the incorporation of microprocessors into some filling systems. The microprocessors make the adjustments necessary for minimizing weight errors on the basis of inputs received from check-weighers, scales, and other sensors which measures the brake and clutch slipage of the auger. Although this technology has improved filling accuracy, the best overall accuracy one can hope to achieve in actual production under ideal conditions (e.g. using a product which exhibits small variations in density) is plus or minus two percent. This means that the packager must adjust the filling machine so that, on the average, it overfills by at least two percent.
Since the auger filling machine dispenses a precise volume for each degree of rotational movement, it is obvious that if the density (weight per unit volume) of the product can be accurately measured, the rotational travel of the auger can be controlled so as to dispense the desired weight.